Formation and geological significance of carbonate firmgrounds and hardgrounds on the Rio Grande Rise (South Atlantic)

Marine hardgrounds are sedimentary rock surfaces formed by early diagenesis. At the Rio Grande Rise, three expeditions documented carbonate firmgrounds and hardgrounds across the central plateaus and the Cruzeiro do Sul Rift flanks and valley. Environmental controls on their formation and preservation in the region remain poorly understood. We investigated 14 carbonate samples from water depths of 641 to 1504 m using petrographic, mineralogical, geochemical, and isotopic analyses. 87Sr/86Sr stratigraphy and microfacies analysis revealed that (i) Neogene wackestones (5.6–2.7 Ma) were deposited in the rift valley under low-energy conditions and cemented during shallow burial diagenesis, (ii) Quaternary rudstones and packstones (1.2–0.7 Ma) occur on the rift flanks, and (iii) Middle Pleistocene to Holocene grainstones (0.5 Ma–Recent) formed on the plateaus under recurrent winnowing and sediment bypass. Cement mineralogy distinguished Type 1 hardgrounds, cemented at or near the sediment–water interface by high-Mg calcite on the flanks and plateaus, from Type 2 hardgrounds, cemented under shallow burial by low-Mg calcite in the rift valley and later exhumed. Plateau samples record a continuum from incipiently cemented firmgrounds to Type 1 hardgrounds. Microfabrics, mineralogy, and carbonate isotopic compositions indicate predominantly marine-phreatic cementation. ROV imagery and hydrodynamic modelling show that bottom-current activity promotes sediment bypass, winnowing, cementation, and variable erosion and deposition on the plateaus, whereas the rift valley retains sediment. We conclude that the rift valley has been a deep-sea depocenter since at least the Late Miocene, whereas the plateaus record post-drowning deep-sea deposition since at least the Middle Pleistocene.